Curtain walls are non-structural walls usually on the outside of a building. We use the term ‘nonstructural’ in the sense that these walls does not carry any load from the building other than their own. Due to this freedom from loading constraints, curtain walls can be made of lightweight materials such as glass, plaster, MDF, or the like instead of iron or concrete as most load-bearing walls are.
In the case of an outer curtain wall, the wall does have to resist wind loads and transfer these to the main building structure. These loads are transferred through connections at floors or columns of the building which hold the curtain wall (such as large panes of glass) in place, through vertical and/or horizontal structures made to hold the curtain walls in place.
Generally, in order to resist direct wind loads, the curtain walls must be connected to the building structure with support members that provide sufficient stiffness to minimize deformations. These support members transfer forces on the curtain wall to the concrete or iron of the building structure, such as at columns or floors of the building.
A typical curtain wall size corresponds with the story height of most office buildings, but when constructing a lobby, penthouse, hall or other structure with high ceilings, the support structures used for typical height curtain walls no longer suffice. For aesthetic effect, the curtain wall preferably spans large heights, in some cases reaching as much as 9 meters in height with single panes of glass.
There is thus a longstanding need for a curtain wall support member adapted to support extraordinarily large curtain walls.
There are many unique challenges for providing such a unique support member. As the window size grows in height and therefore in area, the loads that must be transferred by the support members grow correspondingly, and thus extra-large curtain walls (which may reach several stories in height, with several meters between support members horizontally and possibly surpassing 9 meters in the vertical direction) have extreme requirements for the stiffness of their support members.
To appreciate the challenges of designing a support member of high stiffness values, consider the deformation of a beam under uniform load, as an approximation to the duty that the support members will serve.
This maximum deflection δ as a function of total load P, distance between anchoring points L, modulus of elasticity E, and moment of inertia I is given by
  δ  =      N    ⁢                  PL        3            EI      
The loading P depends on the area of the curtain wall and hence on the first power of L, such that the final dependence of deflection on L is in fact of the fourth power. This causes a dramatic increase of required moment as a function of curtain height; for a curtain of twice the height, the moment required will increase by 2{circumflex over ( )}4 or a factor of 16. High values of L (e.g. 9 meters or more) are in demand, and therefore the moment I must be correspondingly high to keep the deflection to acceptable values. As a practical example, an aluminum support member (of modulus ˜70 GPa) having approximately 7 meters between its own anchoring points (where it is held by concrete, steel, or other structural building elements) will require a moment of over 10,000 cm{circumflex over ( )}4 to keep its maximum deflection δ to an acceptable level of 4 cm.
This challenge is only exacerbated where very high curtain walls that have high L values are desired.
For this purpose, replacing the aluminum material is often attempted in the industry but it also introduces many disadvantages:    (a) Aluminum is a very abundant, easy to produce, and theoretically 100% recyclable material. Replacing aluminum would involve higher production costs resulting from the usage of more valuable and difficult materials;    (b) Aluminum is easily extruded using well-known processes, to produce linear forms of nearly unlimited cross-section design.    (c) Aluminum is a durable and visually appealing material. Replacing aluminum would most often result in a coarse and un-esthetic visual look to the curtain wall. The aesthetic requirement is emphasized where extraordinarily high lobbies are constructed incorporating extra-large curtain walls.
Therefore, there is a need for a solution that would sustain an acceptable deflection δ range and would provide a support member with an acceptable stiffness, without changing the material completely, for example, by use of designs having high moment of inertia I values.
There are various designs for such support members. For the sake of convenience we shall demonstrate these design challenges with a typical prior art I-beam shaped support member (FIG. 3).
As can be seen, the I-beam support member is comprised of 2 relatively thick, parallel, and distant flanges (301a-b) where the glass (304) is mounted upon one of the flanges. The I-beam support member is further comprised of a relatively long web that determines the distance of said flanges, and a holding cap (303) that is responsible for holding the window glass (304) attached to the support member. As will be obvious to one skilled in the art, wind forces (305a-b) will generally cause loads towards the building but will also sometimes tend to pull the windows away from the building due to ‘lift’ of wind perpendicular to the building surface, and therefore the curtain-wall support member must deal with loads in both directions.
As is known by the average person in the art, the most efficient way to increase the moment of inertia of the support member is to increase the web (302) length.
However in the case of extruded members a practical limit of around 30 cm maximum member thickness is reached, since aluminum extruders generally use dies of this diameter and cannot produce items of greater transverse dimensions. While larger dies can be constructed, machines adapted to use them are not generally available and hence this size represents a practical limit in terms of cost of production.
Furthermore, elongating the web, instead of increasing the web thickness (which is not an effective means for increasing the moment of inertia of the member), would create a thin proportioned support member, ultimately degrading the support member's stability by introducing a ‘wobbling’ effect to the web which will lead to further deformation.
Due to these reasons, curtain walls manufacturers not only increase the support member web length (302) but also increase the flanges' (301a-b) thickness that has a secondary impact on the moment of inertia. Nevertheless, designs following this standard design doctrine have practical drawbacks and limitations, for instance namely that unreasonable flange thickness, produces high material requirements and ultimately results in high weight and high materials bill.
Furthermore, due to the 30 cm common extruder limit increasing flange thickness too much comes at the expense of web length, resulting in an inefficient and ineffective disproportionate support member.
Additionally, standard extruders have a practical limit of approximately 130 kg extrusion mass. Therefore, even by disproportionately thickening the support member's flanges, the extruder mass limit will be quickly reached and the required 7 meter long support member with moment of over 10,000 cm{circumflex over ( )}4 will still be unattainable.
To emphasize this point, consider a disproportionately thick flanged support member design having the required moment of 10,000 cm{circumflex over ( )}4. Due to the extruder mass limit, the support member will fall short of the required total length. Under its mass limit, the extruder would only be able to produce a short support member (e.g. 4 meter long member instead of the required 8 meter length) for which much lower moment values suffice.
There is a further practical limitation involving the length and the weight of the extrusions. The transportation and handling of regular curtain wall support members, is already a difficult task and it becomes even more severe, with extra-large support members which are only available through import and naval transportation from countries with suitable massive extruders. In these cases there is a need for a suitable logistical framework and specialized loading and handling methods.
Needless to say that these logistical obstacles greatly increase costs and also prolong delivery times.
Therefore a there is still a long-felt need for a support member that is capable of supporting extra-large curtain walls while not requiring arbitrarily large extrusion sizes and also avoiding transportation and manufacturing related practical limitations